Electrochemical sensor element

ABSTRACT

An electrochemical sensor element ( 10 ) for determining the oxygen concentration in exhaust gases of internal combustion engines is described. The sensor element ( 10 ) has a pump cell, which has a pump electrode ( 52 ) situated on a surface of the sensor element ( 10 ) facing the gas mixture, and a measuring gas electrode ( 50 ) situated in a measuring gas area ( 40 ), the gas mixture entering the measuring gas area ( 40 ) through a diffusion resistor ( 31 ). In addition, the sensor element ( 10 ) has a reference electrode ( 51 ) situated in a reference gas area ( 41 ). Another diffusion resistor ( 32 ) is provided between the measuring gas area ( 40 ) and the reference gas area ( 41 ).

[0001] The present invention relates to an electrochemical sensorelement for determining the concentration of a gas component in a gasmixture, in particular for determining the oxygen concentration andexhaust gases of internal combustion engines according to the preambleof claim 1.

BACKGROUND INVENTION

[0002] Such an electrochemical sensor is known from German PatentApplication 196 47 144 A1, for example. These sensor elements are usedin gas detectors which are used to regulate the air/fuel ratio ofcombustion mixtures in automotive engines and as broadband lambdaprobes. A concentration cell is combined with an electrochemical pumpcell in these sensor elements.

[0003] The concentration cell has a measurement electrode situated in ameasuring gas area and a reference electrode situated in a reference gasarea. The exhaust gas passes through a gas access orifice and adiffusion barrier to enter the measuring gas area. The reference gasarea communicates with a reference atmosphere through an openingsituated on the side of the sensor element facing away from themeasuring gas area. The measuring gas area and the reference gas areaare situated in the same layer plane of the sensor element, which isstructured as a layered system and are separated by a gas-tightpartition. A Nernst voltage develops between the measuring electrode andthe reference electrode and can be used to determine the ratio of theoxygen partial pressure in the measuring gas area to the oxygen partialpressure in the reference gas area.

[0004] The pump cell has a first pump electrode situated in themeasuring gas area and a second pump electrode situated on a surface ofthe sensor element facing the exhaust gas, and it pumps oxygen ions outof the measuring gas area into the exhaust gas, or conversely, out ofthe exhaust gas and into the measuring gas area. A pump voltage appliedto the pump cell is regulated by an external circuit element, so that apredetermined oxygen partial pressure which corresponds to a certainNernst voltage prevails in the measuring gas area. The pump voltage isselected so that the pump current flowing in the pump cell is limited bythe diffusion rate of the oxygen molecules through the diffusionbarrier, and the stream of oxygen molecules flowing through thediffusion barrier is proportional to the oxygen concentration in theexhaust gas, so the oxygen partial pressure of the exhaust gas can bedetermined from the pump current.

[0005] A disadvantage of the known sensor element is that the design oftwo gas spaces that are separated from one another in a gas-tightmanner, namely the measuring gas area and the reference gas area in oneplane of the sensor element is complicated and difficult from thestandpoint of the manufacturing technology.

ADVANTAGES OF THE INVENTION

[0006] The electrochemical sensor element according to the presentinvention as characterized in the independent claim has the advantagethat the structure of the sensor element is greatly simplified byproviding an additional diffusion resistor between the measuring gasarea and the reference gas area. This achieves the result that it is notnecessary to form a recess for the reference gas area separated from themeasuring gas area in a gas-tight manner.

[0007] Advantageous embodiments of and improvements on the sensorelement characterized in the independent claim are possible through themeasures characterized in the dependent claims.

[0008] Since the gas component, generally oxygen, is pumped into thereference gas area by an external circuit element via the referenceelectrode, this achieves the result that a uniform partial pressure ofthe gas component prevails in the reference gas area, so that thepartial pressure of the gas component in the measuring gas area may bedetermined with a good accuracy via the voltage difference (Nernstvoltage) which develops between the measuring gas electrode and thereference electrode.

[0009] It is also advantageous that the reference gas area communicateswith a gas space situated outside the sensor element only via theadditional diffusion resistor. This prevents impurities from thereference gas atmosphere, for example, from entering the reference gasarea, which could result in damage to the reference electrode and wouldthus impair the sensor function.

[0010] The sensor design is greatly simplified further by the fact thatthe measuring gas area and/or the reference gas area is filled in atleast some areas by a porous layer forming the diffusion resistor and/orthe additional diffusion resistor.

[0011] Due to the fact that the reference gas area is situated in achannel-shaped area remote from the gas access orifice, this achievesthe result that the concentration of the gas component in the referencegas area is influenced only slightly by fluctuations in theconcentration of the gas component of the gas mixture.

[0012] Due to the fact that a measuring gas electrode and the referenceelectrode are situated in the same layer plane, this yields themanufacturing advantage that the measuring gas electrode and thereference electrode may be applied in a printing step.

BRIEF DESCRIPTION OF THE DRAWING

[0013] Embodiments of the present invention are illustrated in thedrawing and explained in greater detail in the following description.

[0014] The figures show:

[0015]FIG. 1 a sectional diagram of a first embodiment of the sensorelement according to the present invention along line I-I in FIG. 2;

[0016]FIG. 2 a sectional diagram of the first embodiment of the sensorelement according to the present invention along line II-II in FIG. 1;

[0017]FIG. 3 a sectional diagram of a second embodiment of the sensorelement according to the present invention along line III-III in FIG. 4;

[0018]FIG. 4 a sectional diagram of the second embodiment of the sensorelement according to the present invention along line VI-VI in FIG. 2,and

[0019]FIGS. 5 and 6 longitudinal sections of additional embodiments ofthe sensor element according to the present invention.

DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

[0020]FIGS. 1 and 2 show as the first embodiment of the presentinvention a sensor element 10 of a broadband lambda probe designed as alayered system and having a first, second, third and fourth solidelectrolyte layer 21, 22, 23, 24. A gas access orifice 30 is introducedinto first and second solid electrolyte layers 21, 22. A recesscontaining a diffusion resistor 31, a measuring gas area 40, anadditional diffusion resistor 32 and a reference gas area 41 is providedin second solid electrolyte layer 22. This recess is designed as ashallow cylindrical area, in the middle of which is provided gas accessorifice 30 surrounded by hollow cylindrical diffusion resistor 31 andmeasuring gas area 40 which is also in the shape of a hollow cylinder,and a channel-shaped area which accommodates an additional diffusionresistor 32, directly adjacent to the cylindrical area, and referencegas area 41.

[0021] A ring-shaped measuring gas electrode 50 having a supply lead 50a is provided on first solid electrolyte layer 21 in measurement gaparea 40, and a reference electrode 51 having a supply lead 51 a isprovided in reference gas area 41. A ring-shaped pump electrode 52 isapplied to the outside surface of first solid electrolyte layer 21.Third and fourth solid electrolyte layers 23, 24 are adjacent to secondsolid electrolyte layer 22. A heating element 57 having a heatinginsulation 58 is provided between third and fourth solid electrolytelayers 23, 24.

[0022] Pump electrode 52 together with measuring gas electrode 50 formsa pump cell which pumps oxygen into or out of measuring gas area 40through an external circuit element. The pump voltage applied to thepump cell through the external circuit element is regulated so that apredetermined oxygen partial pressure prevails in measuring gas area 40.An oxygen partial pressure of λ=1 is preferably set, i.e., the oxygenpartial pressure corresponds to the stoichiometric air/fuel ratio.

[0023] The oxygen partial pressure prevailing in measuring gas area 40is determined by a Nernst cell which is formed by measuring gaselectrode 50 and reference electrode 51. A Nernst voltage produced bydifferent oxygen partial pressures in measuring gas area 40 and inreference gas area 41 is measured using the Nernst cell and is used toregulate the pump voltage. This requires that a sufficiently constantoxygen partial pressure prevails in reference gas area 41. Therefore, alow pump current between measuring gas electrode 50 and referenceelectrode 51 or between pump electrode 52 and reference electrode 51 isproduced by the external circuit element.

[0024] Oxygen is pumped into reference gas area 41 by this pump currentof 5 to 50 μA, for example. Depending on the oxygen partial pressure inthe exhaust gas, the oxygen partial pressure in reference gas area 41may vary, for example, in the range of 0.01 bar with rich exhaust gas to0.3 bar with very lean exhaust gas. However, the effect of thesefluctuations on the Nernst voltage is negligible.

[0025] The gas in reference gas area 41 may enter measuring gas area 40through additional diffusion resistor 32. No additional connection to agas space located outside the sensor element, such as the airatmosphere, is necessary. Because of the low pump current, the oxygenpartial pressure in measuring gas area 40 is altered only to anegligible extent by venting of reference gas area 41 into measuring gasarea 40.

[0026] In one conceivable embodiment, reference electrode 51 is mountedon third solid electrolyte layer 23 in reference gas space 41.

[0027]FIGS. 3 and 4 show as the second embodiment of the presentinvention a sensor element 110 of a broadband lambda probe having afirst and a second solid electrolyte layer 121, 122 into which isintroduced a gas access orifice 130, and a third and fourth solidelectrolyte layer 123, 124 between which is provided a heating element157 having a heating insulation 158. Second solid electrolyte layer 122has a recess containing a diffusion resistor 131, a measuring gas area140 having a measuring gas electrode 150, an additional diffusionresistor 132, and a reference gas area 141 having a reference electrode151. A pump electrode 152 is mounted on the outside surface of firstsolid electrolyte layer 121.

[0028] The second embodiment differs from the first embodiment in thatadditional diffusion resistor 132 and reference gas area 141 in the formof concentric hollow cylinders are also situated in a shallowcylindrical recess in addition to diffusion resistor 131 and measuringgas area 140. In addition, reference gas area 141 situated in the areaof reference electrode 151 is filled completely by a porous materialwhich forms additional diffusion resistor 132. Accordingly in thisembodiment, reference gas area 141 is understood to refer to the area ofthe porous material adjacent to reference electrode 151.

[0029] In one conceivable embodiment, the reference electrode inreference gas space 141 is applied to third solid electrolyte layer 123.It is also conceivable for the additional diffusion resistor not tocover reference electrode 151 at all or to cover it only in some areas.

[0030]FIG. 5 shows a third embodiment of the present invention whichdiffers from the second embodiment in that reference electrode 151 inreference gas area 141 is applied to third solid electrolyte layer 123,and another measuring gas electrode 153 is applied to the third solidelectrolyte layer and is opposite measuring gas electrode 150 inmeasuring gas area 140.

[0031] In this embodiment, additional measuring gas electrode 153 may beelectrically connected to measuring gas electrode 150 in the supply leadarea, for example. It is also conceivable for the Nernst cell to beformed by additional measuring gas electrode 153 and reference electrode151 and for the pump cell to be formed by measuring gas electrode 150and pump electrode 152. In this case, the pump current may flow betweenmeasuring gas electrode 150 and additional reference electrode 153and/or between pump electrode 152 and additional reference electrode 153to maintain the required oxygen partial pressure in reference gas area141.

[0032]FIG. 6 shows a fourth embodiment which differs from the secondembodiment in that the recess in the second solid electrolyte layercontaining diffusion resistor 131, measuring gas area 140, additionaldiffusion resistor 132, and reference gas area 141 is filled completelyby a porous material. In this embodiment, measuring gas area 140 isunderstood to be the area of the porous material adjacent to measuringgas electrode 150, and reference gas area 141 is understood to be thearea of the porous material adjacent to reference electrode 151.Diffusion resistor 131 is formed by the porous material situated betweengas access orifice 130 and measuring gas electrode 150, and additionaldiffusion resistor 132 is formed by the porous material situated betweenmeasuring gas electrode 150 and reference electrode 151.

What is claimed is:
 1. A sensor element for determining theconcentration of a gas component in a gas mixture, in particular fordetermining the oxygen concentration in exhaust gases of internalcombustion engines, having at least one pump cell, which has at leastone pump electrode (52, 152) situated on a surface of the sensor element(10, 110) facing the gas mixture, and at least one measuring gaselectrode (50, 150) situated in a measuring gas area (40, 140), the gasmixture entering the measuring gas area (40, 140) through a diffusionresistor (31, 131), and having at least one reference electrode (51,151) situated in a reference gas area (41, 141), wherein an additionaldiffusion resistor (32, 132) is provided between the measuring gas area(40, 140) and the reference gas area (41, 141).
 2. The sensor elementaccording to claim 1, wherein the gas component is pumped into thereference gas area (41, 141) by an external circuit element via thereference electrode (51, 151).
 3. The sensor element according to claim2, wherein the gas component is pumped from the measuring gas area (40,140) into the reference gas area (41, 141) by an external circuitelement via the measuring gas electrode (50, 150) and the referenceelectrode (51, 151).
 4. The sensor element according to claim 2, whereinthe gas component is pumped into the reference gas area (41, 141) by anexternal circuit element via the pump electrode (52, 152) and thereference electrode (51, 151).
 5. The sensor element according to claim1, wherein the reference gas area (41, 141) is only connected to a gasspace situated outside the sensor element (10, 110) by the additionaldiffusion resistor (32, 132).
 6. The sensor element according to one ofthe preceding claims, wherein an additional measuring gas electrode(153) is provided in the measuring gas area (40, 140).
 7. The sensorelement according to claim 6, wherein the additional measuring gaselectrode (153) is electrically connected to the measuring gas electrode(50, 150).
 8. The sensor element according to claim 6, wherein the gascomponent is pumped from the measuring gas area (40, 140) into thereference gas area (41, 141) by an external circuit element via themeasuring gas electrode (50, 150) and the reference electrode (51, 151)and/or via the additional measuring gas electrode (153) and thereference electrode (51, 151).
 9. The sensor element according to claim1, wherein the sensor element (10, 110) is designed as a layered system,and the diffusion resistor (31, 131), the measuring gas area (40, 140),the additional diffusion resistor (32, 132), and the reference gas area(41, 141) are situated at least predominantly in one layer plane of thesensor element (10, 110).
 10. The sensor element according to claim 9,wherein the measuring gas electrode (50, 150) and the referenceelectrode (51, 151) are situated in the same layer plane of the sensorelement (10, 110).
 11. The sensor element according to claim 9, whereinthe diffusion resistor (31, 131), the measuring gas area (40, 140), theadditional diffusion resistor (32, 132), and the reference gas area (41,141) are situated in a cylindrical recess, and the measuring gaselectrode (50, 150) and the reference electrode (51, 151) are designedwith an annular shape.
 12. The sensor element according to claim 9,wherein the diffusion resistor (31, 131) and the measuring gas area (40,140) are situated in a cylindrical recess; the measuring gas electrode(50, 150) is designed with an annular shape; and the additionaldiffusion resistor (32, 132) and the reference gas area (41, 141) aresituated in another recess extending out of the measuring gas area (40,140) in the form of a channel.
 13. The sensor element according to claim1, wherein the diffusion resistor (31, 131) and/or the additionaldiffusion resistor (32, 132) is/are formed by a porous layer.
 14. Thesensor element according to claim 1, wherein the reference gas area (40,140) and/or the measuring gas area (41, 141) is/are filled at leastregionally by a porous layer forming the diffusion resistor (31, 131)and/or the additional diffusion resistor (32, 132).